The invention relates to an improved method of laser doping a semiconductor material. In particular, the invention describes an improved doping source for use in laser doping.
In recent years, laser doping has become a popular method for fabricating large area electronics. The low processing temperatures used in a laser doping process makes the process suitable for doping semiconductor layers on substrates that have low temperature tolerances. Examples of substrates with low temperature tolerances include plastic and glass.
A second reason for the popularity of laser doping is the availability of techniques to make self-aligned amorphous silicon Thin Film Transistors (TFT). Self aligned techniques for forming TFTs utilize the gate electrode as a mask, thereby minimizing alignment problems when forming a passivation island over the gate electrode. Detailed methods for forming such self aligned structure are described in an article by P. Mei, G. B. Anderson, J. B. Boyce, D. K. Fork, and R. Lujan entitled Thin Film Transistor Technologies III published in the Electrochemical Soc. Proc. PV 96-23., p. 51 (1997). U.S. patent application Ser. No. D/97343 entitled xe2x80x9cMethod of manufacturing a Thin Film Transistor with reduced parasitic capacitance and reduced feed-through voltagexe2x80x9d by P. Mei, R. Lujan, J. B. Boyce, C. Chua and M. Hack, also describes fabricating a TFT structure using a self alignment process and is hereby incorporated by reference.
During laser doping, a laser pulse briefly melts a surface layer in a doping region of a semiconductor. While the doping region is in a molten state, a dopant source is introduced near a surface of the doping region. The dopant source provides dopant atoms that are distributed through the surface layer. After the doping region solidifies, the dopants are available for electrical activation.
Typical dopant sources used to provide dopant atoms include: (1) gas dopant sources such as phosphorus fluoride (PF5) and boron fluoride (BF3), (2) spin-on dopants such as phosphorous doped silica solution, and (3) phosphor-silicon alloys which may be ablated from another substrate or deposited directly on the doping region.
Each of the above described dopant sources has a corresponding disadvantage. For example, gas dopant sources are difficult to control and require the use of specialized equipment to prevent uneven distribution of dopants. Furthermore, both gas and spin-on deposition techniques have low doping efficiencies. A low doping efficiency makes it difficult to fabricate a heavily doped material. Phosphor-silicon alloy materials are difficult to deposit and unstable when exposed to moisture in air. In particular, moisture in the air decomposes the phosphor-silicon alloy. After decomposition, the moisture may redistribute the dopant atoms. Dopant atom redistribution produces high defect rates in devices formed from the doped semiconductor.
Ion implantation provides an alternate method of distributing dopant atoms in a semiconductor. However, when used in large area processes, ion implantation requires expensive specialized equipment.
Thus an improved method of laser doping is needed.
Although laser doping has become a popular method of fabricating large area electronic devices, current laser dopant sources are difficult to control, require expensive equipment, or are unstable when exposed to moisture in the air. In order to avoid these problems, the invention describes a technique for using a thin film of dopant containing compound (DCC) as a laser doping source. The dopant containing compound (DCC) can be deposited by standard equipment and is stable in air. An example of a dopant containing compound is phosphorous nitride.
In one embodiment of the invention, standard plasma enhanced chemical vapor deposition (PECVD) equipment is used to deposit a thin phosphorous nitride film over the dopant region. Alternatively, a transparent dopant plate may be used to position the phosphorous nitride near the dopant region in a laser ablation process. In laser ablation, a laser decomposes the phosphorous nitride. The laser also briefly melts the semiconductor in the dopant region to allow incorporation of dopant atoms into the doping region. After the doping process, remaining phosphor nitride may be removed using a plasma containing small amounts of CF4 diluted in oxygen.